1. Field of the Invention
The invention relates to a device and associated method for converting data sequences between a frame relay (FR) format and an asynchronous transfer mode (ATM) format.
2. Description of the Related Art
The asynchronous transfer modus (ATM) serves as a basis for a universal and international broadband communication, whose configuration standardized by means of the ATM forum found in the year 1991. The asynchronous transfer modus is an asynchronous time-division multiplexer method, which enables an integration of different communication services, such as voice communication, data communication, video telephone, video communication etc. given an arbitrary scaling ability of the transmission capacity. The structure of a typical ATM cell is represented in Rathgeb, Wallmeier, page 79, for example.
Network accesses having transmission rates of 64 kbit/s up to 45 Mbit/s are enabled by means of “Frame Relay”, whereby the data frames can have a variable length up to 8 kbytes. FR network accesses are particularly suitable for the communication e.g., in the Internet. The specifications are standardized by means of the FR forum. The structure of an FR frame is described in Rathgeb, Wallmeier, page 269 through page 271, for example.
The conversion of data sequences between FR and ATM format (“interworking”) can be carried out as a “network interworking,” in which the FR frames are directly converted into corresponding ATM frames and vice-versa or can be carried out by means of a “service interworking”, whereby the content of the FR frames are converted into an AAL5 (ATM adaption layer 5) cell stream. The network interworking is described in FR forum document No. FRF. 5 and the service interworking is described in FR forum document No. FRF. 8.
Given the transmission from one format to the other one, the frames must be frequently translated in their control data area (header), particularly regarding the service interworking. For this purpose, the frames of an FR connection that are supplied via the interfaces are stored in the main memory of the central computer (frame processor FP) of the conversion device. For this purpose, the central computer reads in the data from the appertaining interface (such as E1/DS1 in the FR interface module) by means of read commands or by means of a direct memory access (DMA). Subsequent to the processing by means of the central computer, the frames are given to a further processing interface, such as an ATM communication module (segmentation and reassembly sub-layer SAR), by means of write commands or by means of a direct memory access. This method has the disadvantage that the central computer cannot continue to work and that the program running on it must be interrupted during the transmission of the data to and from the main memory of the central computer. Therefore, the throughput rate of the conversion device is significantly reduced. Long FR frames hold up the central computer with waiting times as much as short FR frames intensively load it with respect to the processing time.